It is important that the balancing resistors used in line cards in telephone systems have precise values. Precision resistors permit proper line balancing, and proper balancing causes better voice transmission fidelity and better data transmission accuracy and reliability. Precision resistance values, however, are only one of the factors determining whether or not particular balancing resistors are looked upon favorably by telephone companies.
It is essential that the balancing resistors, and the line cards, perform in certain ways demanded by the telephone companies, at three specified or standard levels of adverse conditions. The first level is one where the line card will survive and continue to function properly despite certain conditions, one being lightning transients. The second level relates to a condition where the balancing resistors are continuously overheating, for example because the line card is overheating due to being improperly connected by a technician. Relative to this second level, there must be a thermal cutoff action to discontinue flow of current before the line-card circuit board starts to burn. The third level is one where there is a sudden application of high voltage, for example when a power line drops on the telephone line. Relative to this third level, current flow must be substantially instantaneously discontinued or small-diameter wires in the telephone system may melt.
The combined requirements for precision balancing, and for the ability to either withstand or fail safely relative to specified types of adverse conditions, must be satisfied by resistors that are physically smaller and smaller in comparison to prior-art resistors, and by resistors having resistance values that are often only a fraction of the values of prior-art resistors. A great problem relative to the needs for physically small balancing resistors, and low-resistance balancing resistors, is that low-resistance resistors have much more power introduced therein during lightning transients and during overload conditions than is the case relative to high-value resistors. For example, if the resistance value is cut in half, the introduced power doubles. Handling high power caused by lightning transients, etc., with physically smaller resistors, is extremely difficult. It is emphasized that physically small resistors inherently have less heat-dissipating surface area and thus are harder to cool in comparison to the physically larger resistors of the prior art.
In sum, telephone companies want the line cards to be small so that there can be more cards in a given space. They therefore want the components on the cards to be small, and they also want the minimum number of components to be employed, while still meeting strict performance requirements.
Relative to the above-mentioned second level of adverse conditions, it is customary to use fuses or thermal cutoffs that are separately manufactured and separately mounted components. These require separate operations to assemble them to the circuit board; furthermore, they require considerable room on the board.
Relative to the above-mentioned third level of adverse conditions, various approaches are employed in the art. These include fuses, voltage-management circuits, etc. These require separate components, and separate mounting operations, as well as needing additional space on the board.